Wavelength division multiplexed (WDM) optical networks increase the information carrying capacity of a communication (e.g., transmission) system by loading multiple channels, each at a different carrier frequency or wavelength, onto a single optical fiber. Over the last few years, the channel density of commercial WDM systems has increased dramatically. At the present time, for example, commercially available systems are available that operate at carrier wavelengths around 1.55 .mu.m and that carry 80 individual channels spaced at 50 GHz. Even larger capacity systems are being planned. These systems are often referred to as dense WDM or DWDM systems. It is advantageous in such systems to use optical sources (or transmitters) that can operate at any one of a subset of the desired channel wavelengths.
However, as these systems are operated for long periods of time, the DBR semiconductor lasers tend to degrade in performance due to aging and material defects. As a result, the wavelength of the laser drifts from the desired channel wavelength. If the drift is sufficiently large, the laser may experience a mode hop; ie., its output may switch abruptly to a different longitudinal mode. In a WDM system, a channel experiencing a mode hop would abruptly start to operate in a mode (i.e., at a carrier wavelength) different from that originally assigned (e.g., at a channel wavelength different from that designated by an ITU grid).
Concomitant with the need to control (i.e., stabilize) the lasers so that each channel operates at a predetermined carrier wavelength (longitudinal mode) without mode hopping is the need to maintain the intensity of other longitudinal modes relatively low, that is, the side mode suppression ratio should be maintained as high as possible.
One prior art approach to achieving wavelength stabilization and maintaining high SMSR is described by S. L. Woodward et al, IEEE Photonics Lett., VoL 4, No. 5, pp. 417-419 (May 1992), which is incorporated herein by reference (hereinafter referred to as Woodward). In the Woodward arrangement a DBR laser includes a Bragg tuning section monolithically integrated with and disposed between a gain section and a photodetector section. A control loop ostensibly ensures single mode operation of the laser with high SMSR More specifically, a 100 kHz sine wave (dither) is added to the tuning current applied to the Bragg section An error signal is derived from light transmitted through the Bragg section to the integrated photodetector. This error signal is detected in a lock-in amplifier and fed back to the tuning section. As shown in FIG. 2 of their paper, Woodward suggests that maximum SMSR is obtained when the first derivative of the detector current (i.e., the laser output power) with respect to the tuning current is zero (i.e., dP.sub.out /dI.sub.tune =0).